Synchronizing system



Ap'nu 1', 1947.

/HETRD D. MITCHELL SYNGHRONIZING SYSTEM Filed June 30. 1943" D. M/TCHE LL BV may@ ATTORNEY Patented Apr. l, 1,947

" sYNcnRoNIzING SYSTEM Doren Mitchell, Bound Brook, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 30, 1943, Serial No. 492,889

5 Claims. l

The present inventionrelatesto a system for synchronizing the movements of separated devices such as may be used, for example, at different terminals of a signaling system.

'I'he invention is particularly suited to cases in which independent drives are provided for the devices at separated stations which are capable of running for short times in suiilciently close synchronism and phase agreement and in which impulses are sent over the system from time to time to correct any accumulated error in the speed of one of the devices to restore synchronism and phase agreement.

An object of the invention is to provide for accurate and reliable speed control of a distant device, including those cases in which the transmission path over which the control is exerted is subject to noise, fading or other diiiiculties.

In accordance with the specic embodiment of the invention to be disclosed herein, the corrective means at afreceiving station is biased to nonoperative condition and this bias is, if anything, increased by presence of noise or during fading so as to guard against false operation. The corrective means remains unoperated except when positively tbrought into action by receipt" of a synchronizing pulse, being at all other times protected against the eiects of spurious energy.

Specifically, a relay is operated, through a noise protection circuit, to throw its armature away from a normal contact to an operate contact to initiate operation of the correcting means. If the relay only partially operates or operates too briefly, the correcting mechanism is not actuated. Full operation of the relay for the duration of the control impulse results in" a comparison between the phase position of the controlled machine and that of the distant machine as determined by the time of receipt of the pulse, and in either no correction or in an advance or a retardation of the phase position of the controlled machine according as the controlled machine is in correct phase or behind or ahead f the distant machine.

A more complete understanding of the invention will be had from the following detailed de scription when read in connection with the accompanying drawing, thesingle ligure of which is a simplified schematic circuit diagram of a complete system embodying the invention.

Referring to the drawing, stations I and 2 are arranged for the transmission of signals between the terminal signal apparatus indicated at I0 and Il, which may be of any desired type. such as telegraph, picture transmission, privacy sys- 55 denser C5 to discharge to ground and thus opv tems or other types of signaling which require the synchronous movement of devices or machines incorporated in or accompanying the signaling apparatus. These signals may employ a band of frequencies passed by the band-pass lters I2 and I3. The line I 4 is illustrative of any type of transmission path or channel and may be a radio link, if desired. These sets of apparatus I 0 and II are driven by respective motors or other driving means indicated at I5 and I6, respectively. On the same shaft with the signaling apparatus at each station is a cam I'l or I8 adapted once each revolution to close corresponding contacts I9 or 20 for a brief instant. These are for purposes of synchronizing the apparatus I Il, I I as will .be described presently.

At station I an oscillator 2| is provided for generating a Wave of frequency F1 outside the .range employed for signaling. A short impulse of this frequency is transmitted over the system each time the springs I9 are closed by rotation of the cam I1 and this impulse is selectively transmitted through the narrow band-pass filter 22.

At the receiving end, this pulse is sent through the first band-pass filter 23 which is arranged to pass a fairly sizable band each side of F1. The output of this lter is then divided, part going through a much narrower filter 24 which passes only about one-fourth the band width of the original filter. 'Ihe output of this narrow filter is fed to the upper amplier detector 25, and the output of the original filter 23, is fed through a band elimination iilter 26, to the lower amplifier detector 2l. The outputs of these two detectors are then fed to the differential relay I, as indicated. When no energy of frequency F1 is present, the output of the amplifier detector 21 will always be greater than that of 25 if any noise is present. This is because the lower amplifier receives about three times the band width of the upper. Thus, at such times, the diii'erential relay I will operate in the down direction. This relay is also biased suiliciently to hold its armature down if no input is present.

When a pulse of frequency F1 is received, however, it will put a comparativelgy large amount of energy in the upper branch of this circuit and Valmost none in the lower (a slight amount due to transients). l Thus, the relay I will operate in the upper direction due to the presence of the pulse. Operation of relay I in the up direction does two things. It releases relay 2 and immediately charges condenser C1 to the full value of the battery 30. Release of relay 2 causes con- When relay 3 operates, however, it will remain operated after relay 2 again operates during the time required to charge up Cs through Re. This time is made equal to a little more than half the time for one rotation of cam Il.

When relay 3 operates, it initiates the operation of relay 4. 'Ihe 'latter relay is made both f slow operate and slow release. This is done by making both resistances Rs and R4 comparatively large. Thus, the relay does not operate until Ce has discharged to a fairly low value through R: but it then remains operated after release of re:

lay I until Cs has been charged up through R4.

Thus, relay 4 is made to operate for a ybrief interval at a time one-half a revolution after the pulse is received from cam I1.

Relay 4, as may be seen, connects the differential detector consisting of tubes V1 and Vz, relay 5 and their associated circuits to the two condensers C1 and Cz. It was pointed out previously that Cr' is charged to its full value every time a pulse is received from the far end.. As soon as this pulse ceases, C1 starts discharging through R1. Cz, however, is also charged to the full value of the same battery through cam I8 every revolution ofthe main drive I6 at this station, and similarly, it starts to discharge through Rz as soon as cam I8 breaks.

If the pulse from the far end occurs at the same instant as the pulse from cam I8, it is apparent that the voltages across R2 and R1 will be equal at all times. Thus, when relay 4 operates for a brief interval, one-half a revolution after the pulse from. cam Il, part of the charge from Cz will be transferred to C4 and a small and equal part of the charge from C1 is transferred t CJ. on Cn, the charge on C: will be equal to that on C4. Thus the plate currents produced in V1 and V: will be equal and differential relay will stay at its mid-position. Thus, the phase of cam I8 will not be changed since the differential drive 3l is not actuated.

Let us now assume, however, that the pulse from cam I'I arrives ahead of the pulse from cam I8. In this case condenser C1 starts to discharge ahead of the time that Cz starts to discharge and thus through the whole cycle the voltage across R1 will be less than that across R2. When relay 4 operates, C4 will be charged to a higher voltage i than C: and the amount by which this voltage exceeds that of Ca will be determined by the amount of time by which the pulse from Cam I1 is ahead ofthe pulse from cam I8.

Thus, the. current in the plate circuit of V1 will exceed that in V2 during the time in which C4 and C: are discharging through R1 and Rs, respectively. Thus, relay 5 will be operated in the up direction for a time dependent on the amount by which the charge in C4 exceeds the charge in Ca. There will, of course, be some low value of current for which relay 5 will not operate in either direction and thus if the difference in volt Since the charge on C1 was equal to that age on Ca and C4 is rather small, the difference currents in the two windings of relay 5 will soon drop to a value too small to operate the relay. On the other hand, if the difference in the voltages on C: and C4 is large, the difference current in relay 5 will remain large enough to operate it for a longer period.

Since the pulse from cam I'I came too soon, the

y voltage in Cz is greater during the remainder of the cycle than that in Ci and thus relay I is operated in the up direction fora time dependent on this diiference in the time of the two pulses. This operates the motor I2 to move the differential gear 3| in the direction to advance the shaft connected to cam I8, This drive is so adjusted that when relay 5 is held operated for a `time determined by the diilerence in arrival of the pulse, operation of the drive for that amount of time changes the phase of cam I8 by very nearly the correct amount to bring cam Il into the right position. If it is not in the correct position when the next pulse comes along, a similar adjustment will, of course, be madel to bring it into more exact synchronism. 1

The above describes what happens when the pulse from cam I I arrives too soon. If it arrives too late, a similar series of events takes place, except that now the voltage on C1 will be greater than that on C: during the discharge ik cyclel and thus relay 5 will be operated in the down direction and the motor 32 will be rotated in the reverse direction so that the phase of cam I8 will be retarded.

As noted above, the time actions in relay l and relay 4 are so arranged that relay 4 operates for a brief interval one-half a revolution after the operation of relay I. It is the diil'erence between voltages on C1 and Cz at the end of this brief interval that determines what happens to relay 5. If cam I8 operates any time from just after the release of relay 4 to just before the operation of relay I, the voltage in C: will be lower than that in C1 the next time relay 4 is operated. This, of course, means that relay 5 `will be operated in the down direction, thus causing cam I8 to be retarded by an amount dependent on the difference in these voltages.

j If cam I8. operates any time from slightly after the operation of relay I to just before the release of relay 4, the voltage in C: will be greater than that in Ci before relay 4 releases, and thus relay 5 will be operated in the up direction for a time dependent on the difference in this voltage and cam I8 will thus be advanced correspondingly.

It is apparent that in the above arrangement no correction is made in the speed at the receiving end unless a pulse gets through to operate relay I. Thus the device is able to, in effect, ignore any period during which no pulse gets through and to maintain its synchronism within the capabilities of its own main drive during that time. As Soon as pulses come through, however, it will immediately pull into synchronism.

What is claimed is: l

1. In a system for synchronizing the motions of distantly located devices, means for periodically sending impulses under the control of the first device, means controlled by the receipt of said impulses for placing a charge of definite value on a condenser, means controlled by the second device for placing an equal charge on a similar condenser, similiar leakage paths for said condensers, a speed corrector for said second device, means for actuating said corrector in accordance with the difference exsiting between the charges on said two condensers, and means actuated by i Q pulses into a voltage diierence, speed corrective means for the second device, means normally preventing said voltage difference from.v reaching said speed corrective means, and means controlled by an impulse produced by the ilrst device and operating only after a delay interval for caus-= ing said voltage diilerenoe to exert a controlling effect on said corrective meansa v In a system for controlling the movement ci a body by impulses sent over a communication channel; means controlled by the. body for periodically charging a condenser, means for` periodically charging another condenser in re sponse to each ci said impulses, a circuit for taking ofi the voltage across both of said condensers, a speed correcting circuit for said body a slow operate relay also controlled by each oi said. im pulses but unresponsive to momentary currents short in comparison to the duration of one of said impluses, and means controlled by said relay when operated for momentarily connecting said speed correcting circuit across the terminals of said inst-mentioned circuitc 4; In a system for controlling the speed of a local mechanism to agree with the speed of a mechanism located at a distance, a transmission path leading from the distant mechanism to the local mechanism, means to send periodic control impulses over said path from the distant mech-4 anism, said path being subject to noise, a narrow band filter and a relatively broad band lter having their inputs connected in parallel to said given rate, a speed correcting circuit adapted to be connected across said condensers to receive the y difference in voltage accross them, and a delayed action relay operated from said differential. relay in response to one of said impulses for momentarily connecting said speed correcting circuit across said condensers after both of said conn densers have had the mentioned charges placed onthem.

5. A system according to claim 4 in which said speed correcting circuit includes condenser y means for storing a charge and resistive circuit means for dissipating the stored charge at a given rate, said delayed action relay in operating transferring the diiferential voltage existing across said ilrstV and second condensers to said con denser means to enable actuation of said cor-frecting means to continue after said delayed action relay has disconnected said correcting cire cuit from across said first and second condensers0 DOREN MITCHELL. 

